The present invention is related to the field of electronic imaging and more specifically relates to methods for fabricating, assembling and optically coupling a multi-element lens system to an image-capture device.
In the field of electronic imaging, digital cameras are constructed using digital image-capture devices such as CCD or CMOS image sensors, and lenses made of glass or plastic materials. High quality digital cameras often have lens systems with variable aperture (iris), variable focal length (zoom) and variable focus. Low cost digital cameras on the other hand often have lens systems with fixed aperture, fixed focal length and fixed focus. Attempts have been made to reduce the cost of such lens systems as well as the cost of assembling and focusing them on low cost digital cameras.
Different methods have been suggested to optically couple the lens system directly to the image-capture device. Optically coupling the lens system to the image-capture device may be accomplished, for example, by gluing the optical element closest to the image-capture device onto its image-sensing surface. One such method is described in international patent application WO 92/15036, the entire disclosure of which is incorporated herein by reference for all purposes. Unlike film cameras where the lens system cannot touch the film (since the film is moved from one picture-taking event to another), digital cameras can be designed with a lens system that touches the image-sensing surface of the capture device.
The benefits of having the lens system touch the image-capture device are numerous. Firstly, the window that usually protects the image-capture device is eliminated. Eliminating this window reduces manufacturing and assembly costs and improves image quality by eliminating unwanted reflections off the surfaces of the window. Secondly, the overall size of the camera can be reduced, especially its thickness, since the lens system can be mounted closer to the image-capture device. This is particularly important for camera modules that are intended for cellular phones, personal digital assistants (PDAs) and laptop computers. In these products, space is a premium and every effort is made to reduce the size of all the components. Finally, optically coupling the lens system to the image-capture device alleviates misalignments, defocusing and other faulty adjustments that can occur if the camera is dropped or mishandled and the lens system is not firmly attached to the image-capture device.
Unfortunately, existing methods for optically coupling the lens system to the image-capture device do not eliminate the laborious step of focusing the lens system onto the image-capture device nor do they reduce the complexity of the lens system. With current methods, the optical combination is not significantly simplified by optically coupling the lens to the image-capture device.
The optical combination is dictated by the overall performance requirement of the camera. Various lens combinations can meet a particular system requirement for field of view, aperture (referred to as f number) and modulation transfer function (referred to as MTF). Typically, a certain number of lens elements are necessary to achieve certain performance levels. For instance, if a narrow field of view is required (i.e., less than 20 degrees) a single element design can usually meet such a requirement, provided that the required aperture is not too large (i.e., a relatively high f number). Such a design can be created with a single plano-convex lens as described in international patent application WO 92/15036. The complexity (and the cost) of the lens increases significantly when the requirement for the field of view increases from 20 degrees to 50 degrees while the aperture is kept relatively high (low f number, e.g. f/3).
Commonly, such lenses are made of four to six elements of different glass materials to correct for chromatic aberrations. A selection of such lenses can be found in the 2000 Edmund Scientific catalog entitled “Electronic Imaging Components” on pages 50 to 55. As the number of elements increases, the material cost and assembly cost of the lens system both increase.
Attempts have been made to reduce the number of lens elements by using aspherical elements, specifically injection-molded plastic aspherical elements. Plastic lens elements are well suited for certain applications, such as disposable film cameras where the image area is very large (800 mm2) and the spatial resolution is fairly low (20 lp/mm). They do not work well, however, with the latest generation of CCD and CMOS capture devices which have a photosensitive area of 4 mm2 and pixels as small as 3.2 μm×3.72 μm (e.g., Sony ICX238AKE). The pixel size has been intentionally shrunk to less than 4 μm×4 μm in an effort to reduce the cost of the silicon chip. In order to resolve such small pixels, the lens system must have a good contrast at spatial frequencies in excess of 125 line pairs per millimeter (“lp/mm”). This in turn implies a surface quality better than λ/4 for the lens elements. Such high surface quality can be achieved through glass polishing. In contrast, the industry-standard surface quality for plastic lenses is only 2.5λ (ten times worse) for reasons detailed in the article entitled “An Introduction to the Design, Manufacture and Application of Plastic Optics” by Michael Missig et al. from OCLI company, the entire disclosure of which is incorporated herein by reference for all purposes.
For the foregoing reasons, while plastic lenses might seem attractive because of their low manufacturing cost, they are not suitable for applications involving image-capture devices with small pixels. For such applications, harder materials such as glass, quartz, rutile, ruby, fused silica or other such materials are preferred. For the sake of brevity, all such materials will be referred to herein as “glass,” whether the material is a true glass or has crystal structure. Prior art multi-element lens systems using such materials can offer an acceptable level of performance, though at a high cost. It would be therefore desirable to have an inexpensive, simple and reliable method of fabricating, assembling and optically coupling multi-element glass lens systems to image-capture devices.